1. Field of the Invention
The present invention relates to an improvement of a pneumatic nozzle for false-twisting a bundle of fibers in the spinning process for producing a fasciated yarn.
2. Description of the Prior Art
In fasciated yarn spinning, a bundle of fibers (hereinafter referred to as a fiber bundle) delivered from a pair of front rollers of a draft means is introduced into an air nozzle and therein is false twisted by a vortex. During untwisting of the false twisted fiber bundle, part of the fibers in the fiber bundle entangle around a core portion thereof to form a fasciated yarn. Generally speaking, in order to obtain a good quality yarn, the fiber bundle has to be fed in the shape of a ribbon having a sufficient number of free-end fibers prior to being twisted. A free-end fiber is a fiber wherein one end thereof is embedded in the body of the bundle and the other end is free. By the application of a vortex, part of the free-end fibers are converted to wrap fibers which entangle around the core portion of the resultant yarn.
In order to generate a sufficient number of free-end fibers, twisting of the fiber bundle in the nozzle inlet must be suppressed as much as possible to maintain the ribbon shape thereof. Therefore, a typical conventional air nozzle has a small channel in its fiber passage between the inlet portion and a large channel in which a vortex is generated. The false twist of the fiber bundle can be prevented to a certain extent from ascending to the inlet portion by the small channel. For example, Japanese Unexamined Patent Publication (Kokai) No. 52-63462 discloses such a type of pneumatic nozzle for producing a fasciated yarn. In such a type of nozzle, the twist-suppressing effect increases as the cross-sectional size of the small channel becomes smaller. However, a reduction of the size of the small channel causes insufficient suction of the air nozzle, which results in various problems, such as the generation of many flies, fiber wrapping on the roller surface, and the generation of less free-end fibers. On the other hand, if the cross sectional area of the small channel is enlarged to increase the quantity of air sucked from the inlet portion of the nozzle so as to prevent the above-mentioned trouble, the twist-suppressing effect by the small channel is reduced. Accordingly, effective fasciation of fibers in the resultant yarn cannot be expected. Moreover, there is a certain possibility to create a ballooning of the fiber bundle in the nozzle, which obstructs the twisting and untwisting phenomenon of fiber bundle in the nozzle. To eliminate these drawbacks of the prior art nozzle disclosed, it was considered to increase the volume of air sucked into the inlet portion of the nozzle while keeping the cross-sectional area of the small channel restricted in size, by which the twist suppressing effect can be maintained in a desirable condition. The air introduced into the large channel is discharged therefrom rapidly by shortering it so as to maintain the air pressure in the large channel as low as possible. However, in such a condition, the twisting and untwisting operation of the fiber bundle in the nozzle cannot be effectively carried out and the above-mentioned ballooning of the fiber bundle in the nozzle becomes more active so that the running condition of the fiber bundle in the nozzle becomes unstable. Consequently, frequent yarn breakages cannot be prevented.
Examined Patent Publication (Kokoku) No. 56-31370, discloses a false twisting nozzle having a short axial length thereof which is comparatively shorter than the conventional nozzle. This nozzle is connected to a tube for untwisting the bundle of fibers twisted by the action of the nozzle. The inside diameter of this tube is made smaller than the outlet diameter of the nozzle. A plurality of fine grooves is formed in the inside wall of the tube so as to enhance the discharge of air therefrom. Such fine grooves in the inside wall, however, damage fibers in the outer layer of the bundle of fibers.